Efficiency and scaling of current drive and refuelling by spheromak injection into a tokamak

The first measurements of current drive (refluxing) and refuelling by spheromak injection into a tokamak are discussed in detail. The current drive mechanism is attributed to the process of helicity injection, and refuelling is attributed to the rapid incorporation of the dense spheromak plasma into the tokamak. After an abrupt increase (up to 80%), the tokamak current decays by a factor of three because of plasma cooling caused by the merging of the relatively cold spheromak with the tokamak. The tokamak density profile peaks sharply because of the injected spheromak plasma (n[sub]e increases by a factor of six) and then becomes hollow, suggestive of an interchange instability. Also discussed is the energy efficiency of spheromak injection current drive and the scaling of this process to larger machines. Refuelling by spheromak injection appears to be a viable scheme for larger machines. However, refluxing by spheromak injection is limited by geometrical and electrical efficiencies (both about 10%) as well as a high repetition rate requirement

Simple, high current LaB_6 cathode

A cathode constructed of a thin, directly heated strip of LaB_6 is described. The cathode is simple to construct, requires modest heating power, has high current emission capability and is quite rugged. Construction details will be given and cathode performance data presented. The cathode has been used in tokamak dc current injection experiments

Motion and equilibrium of a spheromak in a toroidal flux conserver

A number of experiments have been performed on spheromaks injected into the empty vacuum vessel of the Caltech ENCORE tokamak (i.e., without tokamak plasma) [Phys. Rev. Lett. 64, 2144 (1990); Phys. Fluids B 2, 1306 (1990)]. Magnetic probe arrays (in a number of configurations) have been used to make single shot, unaveraged, in situ measurements of the spheromak equilibrium. These measurements are important because (i) they reveal for the first time the equilibrium structure of spheromaks in a toroidal geometry, (ii) they provide a reliable estimate of magnetic helicity and energy of spheromak plasmas used in injection experiments [Phys. Rev. Lett. 64, 2144 (1990)], and (iii) they constitute the first measurements of spheromak motion across and interaction with static magnetic fields (which are useful in corroborating recent theories). Probe measurements in the tokamak dc toroidal field show for the first time that the spheromak exhibits a ``double tilt.''The spheromak first tilts while in the cylindrical entrance region, emerging into the tokamak vessel antialigned to the dc toroidal field, then expands into the tokamak vacuum vessel, and finally tilts again to form an oblate (nonaxisymmetric, m=1) configuration. In addition, the spheromak drifts vertically in the direction given by Jcenter×Btok, where Jcenter is the unbalanced poloidal current that threads the center of the spheromak torus. Probe arrays at different toroidal locations show that the spheromak shifts toroidally (horizontally left or right) in the direction opposite that of the static toroidal field. In the absence of toroidal flux, the m=1 object develops a helical pitch, the sense of the pitch depending on the sign of the spheromak helicity. The spheromak equilibrium in the toroidal vessel is well fit by a pressureless infinite cylindrical model; however, there is evidence of deviation from m=1 symmetry because of toroidal effects, nonuniform J/B profile, and finite beta. Experiments performed in a test facility consisting of the spheromak gun and a replica of the entrance region (with a closed end) show that the spheromak is generated with its axis coaxial with that of the gun. Coherent, m=2 magnetic modes are observed during the formation stage rotating in the E×B direction at about 125 kHz (rotation velocity corresponding to 40% of the Alfvén speed)

Physical Custody in Wisconsin Divorce Cases, 1980–1992

This report examines data from the court record of 9,500 Wisconsin divorce cases in twenty-one representative counties for a period of twelve years, from 1980 to 1992, in order to document how child custody is being handled in divorce. We find an increased involvement of fathers with their children after divorce, particularly through joint legal custody (81 percent in 1992), but also in shared physical custody (14 percent in 1992), and in an increase in specific and detailed physical placement awards. We find substantial differences in custody awards related to situations and actors in the divorce process, and wide variations in custody awards between counties. We also find major differences between families who have equal 50/50 time-share and those who have unequal shared-custody arrangements. These two kinds of shared-custody cases have been treated as one type in the research on custody to date, but appear in our data to characterize two quite different kinds of families.

The local extragalactic velocity field, the local mean mass density, and biased galaxy formation

The biased galaxy formation picture accounts for the low apparent mass density derived from clustering dynamics by the assumption that the mass per galaxy is unusually low in the regions of high density where clustering has been studied. It would follow that the mass per galaxy is unusually high where the mass density is low, and, by continuity, that the mass per galaxy is close to the global mean in regions where the ambient mass density, p_t, is close to the global mean, P_b. That is, we would expect that the best chance for an unbiased estimate of the mean mass per galaxy, and hence of P_b, would be from the dynamics of regions with p_t ≈ P_b. The local density at redshifts 200 ≾ cz ≾ 400 km s^(-l) must be close to P_b because, as Sandage has emphasized, the local Hubble flow is so little perturbed. In this paper we derive a relationship between the local mass density and the perturbation of the local Hubble flow. The local mass density is estimated by the method used in the Virgocentric flow. We use the infrared Tully-Fisher distances of Aaronson et al. to find limits on the gravitational perturbation to the local Hubble flow, and we use bright galaxy counts, N, to estimate the local galaxy concentration. The statistics on the latter are weak because N is small. We can conclude, however, that if mass were proportional to N, with no fluctuations, and the local mass per galaxy were a fair sample, then the density parameter (Ω = P_b/Einstein-de Sitter density) would be Ω ≈ 0.1, consistent with the other dynamical estimates and inconsistent with the above naive interpretation of biasing

Force platform recordings in the diagnosis of primary orthostatic tremor

Primary orthostatic tremor (OT) consists of rhythmical muscle contractions at a frequency of around 16 Hz, causing discomfort and/or unsteadiness while standing. Diagnosis has hitherto relied on recording Electromyography (EMG) from affected muscles. The main aim of this study was to see if the characteristic postural tremor in OT can be identified with force platforms. We also quantified postural sway in OT patients to assess their degree of objective unsteadiness. Finally, we investigated the time relations between bursts of activity in the various affected muscle groups. Subjects stood on a force platform with concurrent multichannel surface EMG recordings from the lower limbs. Seven patients with clinical and EMG diagnosis of OT were examined and the force platform data compared with those of 21 other neurological patients with postural tremor and eight normal controls. All OT patients had high frequency peaks in power spectra of posturography and EMG recordings (12–16 Hz). No such high frequency activity was evident in patients with Parkinson's disease, cerebellar degenerations, essential tremor or in healthy controls. Additionally, OT patients showed increased sway at low frequencies relative to normal controls, suggesting that the unsteadiness reported by OT patients is at least partly due to increased postural sway. Examination of EMG timing showed fixed patterns of muscle activation when maintaining a quiet stance within but not across OT patients. These data show a high correlation between EMG and posturography and confirm that OT may be diagnosed using short epochs of force platform recordings

Evolution of virulence in opportunistic pathogens: generalism, plasticity, and control

Standard virulence evolution theory assumes that virulence factors are maintained because they aid parasitic exploitation, increasing growth within and/or transmission between hosts. An increasing number of studies now demonstrate that many opportunistic pathogens (OPs) do not conform to these assumptions, with virulence factors maintained instead because of advantages in non-parasitic contexts. Here we review virulence evolution theory in the context of OPs and highlight the importance of incorporating environments outside a focal virulence site. We illustrate that virulence selection is constrained by correlations between these external and focal settings and pinpoint drivers of key environmental correlations, with a focus on generalist strategies and phenotypic plasticity. We end with a summary of key theoretical and empirical challenges to be met for a fuller understanding of OPs

Information Asymmetry and Foreign Currency Borrowing by Small Firms

We model the choice of loan currency in a framework which features a trade-off between lower cost of debt and the risk of firm-level distress costs. Under perfect information foreign currency funds come at a lower interest rate, all foreign currency earners as well as those local currency earners with high revenues and/or low distress costs choose foreign currency loans. When the banks have imperfect information on the currency and level of firm revenues, even more local earners switch to foreign currency loans, as they do not bear the full cost of the corresponding credit risk.foreign currency borrowing;competition;banking sector;market structure